1. Field of the Invention
The present invention relates to wavelength-multiplexing optical communication technology for transmitting the data of a plurality of channels over a single optical fiber by the use of a plurality of wavelengths.
2. Description of the Related Art
In recent years, there have been incarnated optical communication systems in each of which digital data are transmitted by performing optical amplification and optical relay or repeat with an optical fiber such as EDF (erbium-doped fiber). In the conventional system wherein signal light of only one wave is optically amplified and relayed, the digital data are transmitted by optimally managing dispersion in such a way that the transmission coding format of the system is set at the mode of an NRZ (non-return-to-zero) signal, and that a negative dispersion wavelength is employed using a DSF (dispersion shift fiber) as a transmission line. First, these basic techniques will be explained below.
As illustrated in FIG. 1, a zero-dispersion wavelength is inherent in an optical fiber. It is a wavelength at which the delay time of a signal becomes 0 (zero). In the DSF which is common as the optical fiber for transmitting signal light, the zero-dispersion wavelength is 1.5625 .mu.m!. That dispersion characteristic of the optical fiber which indicates the relationship of the delay time of the signal on this optical fiber to the wavelength of the signal light, exhibits a quadratic curve whose minimum value lies at the zero-dispersion wavelength as shown in FIG. 1. For the DSF, a wavelength on the order of 1.5585 .mu.m! shorter than the zero-dispersion wavelength is used as the signal light wavelength. In this case, the dispersion characteristic in the vicinity of the signal light wavelength becomes the negative one in which the delay time decreases with increase in the wavelength of the signal light and increases with decrease in the same. The dispersion value of the signal light involved here is -0.3 psec/nm/km! (picosecond/nanometer/kilometer) or so at the signal light wavelength. That is, in a case where the signal light having the signal light wavelength mentioned above is transmitted 1000 km! over the DSF, the signal light delays -300 psec! on condition that the wavelength of the signal light has increased 1 nm! from the specified signal light wavelength.
Meanwhile, in a case where signal light obtained by directly modulating a digital signal as shown in FIG. 2 is transmitted over an optical fiber, the signal light is not always transmitted at the signal light wavelength thereof, but it is transmitted with a predetermined dispersion centering on the signal light wavelength as illustrated in FIG. 3.
Accordingly, when the signal light is transmitted centering on the signal light wavelength as stated above, over the optical fiber such as DSF, the arrival rates of light come to differ at the respective components of wavelengths which constitute the signal light centering on the signal light wavelength. Consequently, degradations in the signal light, such as a dull or blunt waveform, occur on a reception side for receiving the transmitted signal light.
In order to transmit the signal light without any delay, theoretically the signal light may be transmitted at the zero-dispersion wavelength of the optical fiber. The DSF, however, has the property that noise is maximized at the zero-dispersion wavelength by 4-light wave mixing or multiplexing or by a nonlinear effect called the "optical parametric effect". It also has the property that noise is abruptly amplified at a positive dispersion wavelength by a nonlinear effect called the "modulation instability". Therefore, the signal light is ordinarily transmitted at a negative dispersion wavelength.
In this regard, a technique called the "dispersion management" has heretofore been employed for compensating for the signal delay at the negative dispersion wavelength. The dispersion management is the technique in which a DCF (dispersion compensation fiber) being a single-mode fiber is inserted every fixed length of the DSF, whereby the delay time is compensated for so as to become 0 (zero) within a predetermined wavelength range centering on the signal light wavelength as illustrated in FIG. 4.
More concretely, referring to FIG. 5, the signal light has the spread of wavelengths as denoted by mark (*) at and near the signal light wavelength on the DSF. In this case, as stated before, the zero-dispersion wavelength of the DSF is 1.5625 .mu.m!, and the signal light wavelength is on the order of 1.5585 .mu.m!, so that the dispersion characteristic which indicates the relationship of the delay time of the signal to the wavelength of the signal light becomes the negative one. On the other hand, the zero-dispersion wavelength of the DCF is on the order of 1.31 .mu.m!, and the specified signal light wavelength 1.5585 .mu.m! is longer than this zero-dispersion wavelength 1.31 .mu.m! of the DCF. As illustrated in FIG. 5, therefore, the dispersion characteristic of the DCF at the particular signal light wavelength becomes the positive one in which the delay time increases with increase in the wavelength of the signal light and decreases with decrease in the same. Owing to the insertion of the DCF every fixed length of the DSF, accordingly, the negative dispersion characteristic based on the DSF is cancelled by the positive dispersion characteristic based on the DCF, and the compensation for zeroizing the delay time is realized in the predetermined wavelength range centering on the signal light wavelength. In this case, the dispersion value of the signal light at and near the particular signal light wavelength on the DSF is approximately -0.3 psec/nm/km!, while the dispersion value of the signal light at and near the particular signal light wavelength on the DCF is approximately +20 psec/nm/km!. Therefore, the length of the DCF to be inserted may be short as compared with that of the DSF.
On the ground of the optical amplification/relay technique explained above, a wavelength multiplexing system wherein the digital data of a plurality of channels are transmitted with a plurality of wavelengths over a single optical fiber is recently intended for practical use due to an increased data transmission capacity, in such a way that only the construction of each terminal station is altered with the existing transmission line diverted, by utilizing a bit-rate-free property which is one of the advantages of the optical amplification/relay system.
In incarnating the wavelength-multiplexing transmission system on the existing transmission line, however, problems to be explained below are involved: